1. Field of the Invention
The present invention relates to a urethane prepolymer composition and a polyurethane coating composition system. More particularly, the present invention is concerned with a urethane prepolymer composition which comprises in specific weight proportions a terminal isocyanate group-containing urethane prepolymer derived from a polytetramethylene glycol having a specific molecular weight and a terminal isocyanate group-containing urethane prepolymer derived from a terminal hydroxyl group-containing polycaprolactone having a specific molecular weight. Also, the present invention is concerned with a polyurethane coating composition system which comprises the above-mentioned urethane prepolymer composition and a compound having at least two hydroxyl groups. The urethane prepolymer composition and the hydroxyl group-containing compound are separately provided and adapted to being mixed when used. When the urethane prepolymer composition and the hydroxyl group-containing compound are mixed and applied to a substrate, polyurethane formation reaction occurs, and a coating film having not only excellent transparency, elastic recovery and weathering resistance but also excellent low-temperature physical properties, e.g., mechanical strength, flexibility and impact resistance at low temperatures, is produced. Especially advantageous application of the present polyurethane coating composition system is found in the coating of resin products, such as polyurethane bumpers and facing materials for buildings.
2. Discussion of Related Art
The use of a resin product in place of a metal product, such as a steel plate, has been increasing in recent years in various fields. For example, this is notable in the field of automobile parts. The use of resin-made automobile parts, e.g., a polyurethane bumper, is remarkably increasing due to their advantages in safety, light weight, shock absorption, etc. However, generally, the resin product is susceptible to discoloration and performance lowering by outdoor exposure for a prolonged period of time. To alleviate the discoloration and performance lowering of a resin substrate, a coating composition is usually applied to the resin substrate. The coating composition is required to be capable of forming a coating film resistant to severe conditions to which for example, an automobile is exposed, and also required to be capable of forming a coating film having not only excellent weathering resistance, elastic recovery and adhesion but also excellent low-temperature physical properties, e.g., mechanical strength, flexibility and impact resistance at low temperatures, e.g., -30.degree. C.
With a view toward developing such a desirable coating composition, various proposals have been made. For example, it was proposed in Japanese Patent Application Publication Specification No. 48-32563 to use a one-pack coating composition of baking type which comprises a polyurethane resin having a terminal hydroxyl group and an aminoplast resin and which is baked to effect crosslinking of the resin. However, this coating composition has drawbacks in that baking must be performed at high temperatures, and in that the coating derived from the composition has poor flexibility at low temperatures.
In Japanese Patent Application Laid-Open Specification No. 58-32662, it was proposed to add a polycaprolactone diol or polyol as an additive to the so-called two-pack polyurethane coating composition system which is suited for use in coating a steel plate body for an automobile but poor in elastic properties of the coating derived from the composition, so as to provide a coating composition system capable of forming a coating having improved elastic properties so that it can advantageously be used for coating a resin product, such as a polyurethane bumper. In this coating composition system, in addition to the diol or polyol to be used as a main component (such as an acrylic polyol and a polyester diol or polyol) and the so-called curing agent (such as an isocyanurate trimer of 1,6-hexamethylene diisocyanate, a biuret adduct of 1,6-hexamethylene diisocyanate, and an isocyanurate trimer of isophorone diisocyanate), a polycaprolactone diol or polyol is employed as an additive. Since the additive also has a hydroxyl group, it is necessary to adjust the ratio of the main component diol or polyol to the curing agent in accordance with the amount of the additive so as to maintain the NCO/OH ratio at an appropriate value. This is not facile and is likely to cause the NCO/OH ratio to be inappropriate. In addition, the polycaprolactone diol or polyol is effective for improving the elastic properties at low temperatures of the coating derived from the composition, only when the diol or polyol is added in a large amount. However, the addition of the polycaprolactone diol or polyol in a large amount is likely to cause the coating derived from the composition to have poor weathering resistance.
In Japanese Patent Application Laid-Open Specification No. 58-59213 and Japanese Patent Application Publication Specification No. 62-53525, it is described that a linear polyurethane elastomer produced by reacting a copolymer diol or polyol derived from a polytetramethylene glycol and a caprolactone with a diisocyanate has excellent hydrolysis resistance and low-temperature physical properties. However, this linear polyurethane elastomer cannot be used in fields where the coating film must have a high level of resistance to weathering, solvents, etc.
Japanese Patent Application Laid-Open Specification No. 61-12753 discloses a urethane prepolymer obtained by reacting a polycaprolactone diol or polyol having a specific number average molecular weight with an aliphatic or alicyclic diisocyanate. The specification also discloses that when the above-identified prepolymer is mixed with an acrylic polyol, a polyurethane coating composition system can be obtained, in which even though the acrylic polyol has a relatively high glass transition temperature, the polyol has good compatibility with the prepolymer, so that the polyurethane coating composition system can provide a coating film having high elongation and excellent weathering resistance. However, the coating film disclosed in the specification does not have desirable low-temperature physical properties. For example, it has poor elongation at low temperatures, such as -30.degree. C.
Japanese Patent Application Laid-Open Specification No. 54-60395 discloses a urethane prepolymer produced by reacting a polytetramethylene glycol with a diisocyanate at an NCO/OH equivalent ratio of 2. However, this prepolymer disadvantageously has high viscosity so that the processing thereof is difficult. In addition, the prepolymer has poor compatibility with a diol or polyol.
As apparent from the foregoing, all of the proposed prepolymers and coating composition systems prepared using the same have problems. The use of a resin product in place of a metal product, such as a steel plate, has been increasing in various fields in recent years. Therefore, there is still a strong demand in the art for a prepolymer which is useful for preparing a two-pack coating composition system which can be advantageously utilized for protecting and providing a good appearance to resin products.